Radial Compression Mechanism

ABSTRACT

A radial compression mechanism utilizes a string wrapped around a plurality of compression dies to move the dies inward and close a central cylindrical cavity defined by the working surfaces of the dies. The string may be coupled to a string tension mechanism that enables a user to applied a desired tension to the string and thereby compress an article within the central cylindrical cavity. The compression dies may be coupled to a base and move along die-guiding slots from an open position to a closed position. A spring may be configured to force the compression dies open and provide some back-tension to the string. The string may extend around a pully on an opposing side of the compression mechanism and both ends of the string may be coupled to the string tension mechanism.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/491,188 filed on Sep. 5, 2019 and currently pending, which is anational stage application of POT application No. PCT/US2019/015015,having an international filing date of Jan. 24, 2019 and now expired,the entirety of both applications are hereby incorporated by referenceherein.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to radial compression mechanisms and morespecifically to mechanisms for compressing devices such as stents,catheters, balloons, and the like.

Background

In the manufacture and testing of medical devices, mechanisms are usedto radially compress cylindrical devices such as stents, balloons, andcatheters. For example, installation of a stent onto a catheter balloonis typically done by compressing the stent radially inward onto theballoon with enough pressure to permanently deform the stent to asmaller diameter and to slightly embed the metal stent into the plasticballoon. In another example, a polymer catheter balloon is compressedradially after pleating to wrap it tightly around the catheter shaft. Inanother example, a self-expanding stent is radially compressed to insertit into a sheath or delivery system. In another example, a stented, ormetal-framed, prosthetic heart valve is compressed radially to deformand reduce the valve's metal structure and assemble it together with adelivery balloon catheter.

For some types of devices, such as prosthetic heart valves that includeparts made from animal tissue, the crimping process is usually performedin the hospital's operating room, just prior to use of the medicaldevice.

Large-diameter stents and balloons, such as aortic stents or prostheticheart valves tend to require a large amount of radial force during thecrimping process. The radial force applied to the product is equal tothe surface area of the crimped product multiplied by the surfacepressure. For example, a typical coronary stent is crimped to a size ofabout 1 mm diameter and 20 mm length while a typical prosthetic aorticheart valve is crimped to a size of about 6 mm diameter and 20 mmlength. The heart valve has about six times more surface area than thecoronary stent. Therefore, to reach the same surface pressure, about sixtimes the radial force would be required.

For radial compression mechanisms that are actuated by human hand, or bya linear actuator such as an air cylinder, we can describe a “mechanicaladvantage” as the ratio of radial force applied to the processed productdivided by the actuation force applied by the hand or actuator.

A first type of prior art device includes a radial compression mechanismwherein several similar wedge-shaped dies with planar surfaces arearranged to form an approximately cylindrical central cavity, the wedgesbeing hinged and driven in unison to change the diameter of the cavity.An example of this mechanism is Machine Solutions Incorporated'shand-operated HV500 crimper, which, because of its low-costinjection-molded construction, should be suitable as a one-use,disposable, stent or prosthetic heart valve crimper for use in a sterileoperating room. However, there are significant disadvantages to thiscrimper. The hinging, or pivoting dies have small pins that support thedies and transmit the forces, so the concentrated mechanical stressreduces the radial force capability of the device. The radial force isalso limited because the handle moves through only a small distance ofabout 45 mm as the opening reduces from about 30 mm to about 6 mm,resulting in a very low mechanical advantage, and therefore inadequateradial force and pressure applied to the stent or heart valve.

A second type of prior art device includes a radial compressionmechanism wherein several similar radially-movable dies withinward-facing surfaces are arranged to form an approximately cylindricalcentral cavity, the dies being constrained to move along radial lines,and being driven in unison by a rotating camming plate with pin/slotengagement to the dies, to change the diameter of the cavity. An exampleof this type of mechanism is described in U.S. Pat. No. 7,530,253B2 andis sold by Edwards Lifesciences Corporation as an injection molded,disposable prosthetic heart valve crimper for use in a sterile operatingroom. The use of a camming plate that rotates though a large,approximately 200 degrees, angle results in a mechanical advantage thatis improved over the Machine Solutions crimper. However, furtherimprovement in the mechanical advantage would be advantageous and wouldallow heart valves to be crimped to higher radial force, or pressure,resulting in a smaller size that is easier and less invasive for valveimplantation procedures. Another shortcoming of this these crimpingdevices is that the radial force must be transmitted through therelatively small contact area between the slots of the camming plate andpins of the dies, which also limits the device's radial forcecapability.

There are many other examples of radial compression mechanisms, orcrimpers, in which the radial forces must be transmitted through smallpins and/or ball bearings that roll on cam surfaces, and in which theradial force that may be applied to the processed device is limited bythe force that can be safely transmitted through the cam-following pins,bushings, or ball bearings.

It would be highly advantageous, therefore, to remedy the foregoing andother deficiencies inherent in the prior art.

SUMMARY OF THE INVENTION

An exemplary radial compression mechanism utilizes a string wrappedaround a plurality of compression dies to move the dies inward and closea central cylindrical cavity defined by the working surfaces of thedies. The string may be coupled to a string tension mechanism thatenables a user to applied a desired tension to the string and therebycompress an article within the central cylindrical cavity. Thecompression dies may be coupled to a base and move along die-guidingslots from an open position to a closed position. A spring may beconfigured to force the compression dies open and provide someback-tension to the string. A plurality of springs may be configuredbetween the plurality of compression dies and they may be retained inposition by spring seats configured on the compression dies, such as apost or recess.

An exemplary radial compression mechanism comprises a plurality of diesarranged in circular array around a central cavity. An exemplary radialcompression mechanism may have a number of compression dies including,but not limited to, three or more, five or more, seven or more, ten ormore, fifteen or more and any range between and including the number ofdies provided. Each of the plurality of dies comprises an inwardlyfacing working surface and these working surfaces define the centralcylindrical cavity. The working surfaces move with respect to each otherand may touch and slide across each other or have a very small gaptherebetween, such as no more than 1 mm, no more than 500 um, no morethan 250 um, no more than 100 um, no more than 50 um, no more than 25 umand any range between and including the gap dimensions provided. Thecircular array of compression dies forms an outer perimeter along theouter surfaces of the dies. The string contacts the outer surfaces tomove the dies inward.

An exemplary base of a radial compression mechanism comprises adie-guiding feature or slot to control the motion of compression dies. Adie-guiding slot may be an aperture in a base member, a recess in a basemember or a pair of rails or protrusions forming a slot therebetween. Anexemplary base member may be configured on opposing sides of thecompression dies and have corresponding die-guiding slots in each. Itshould also be noted that the system for guiding the die motion may beachieved with a wide range of design elements, including, but notlimited to: 1) tabs or pins on dies engaging slots in stationary plates,as shown above, or 2) tabs or pins on the stationary plates engagingslots on the dies, or 3) cam-follower type ball bearings or plainbearings, engaging cam surfaces, or 4) ball bearings or plain bearingsupon which the dies are mounted directly, so that the hinging point forthe dies' motion is the bearing itself.

An exemplary string is a supple elongated member that such as a cord,rope, string, band and the like. In some embodiments however, a stringmay have a rectangular or irregular cross-sectional shape. An exemplarystring may be made out of low friction materials, such a polymer such asplastic including but not limited to polyethylene, ultra-high molecularweight polyethylene, such as Spectra fiber available from HoneywellInternational Inc., nylon, fluoropolymer including fluorinated ethylenepropylene (FEP), polytetrafluoroethylene, metal string or strands,stainless steel, and the like. An exemplary string may comprise aplurality of strands that are twisted, wrapped, or braided around eachother or braided. An exemplary string may be a core-sheath type stringhaving a core of one type of material and a sheath of another type ofmaterial. Note that low friction coatings or lubricants may be used withthe string or applied to the string and/or the outer surface of thecompression dies.

An exemplary string may be wrapped around the outer perimeter of thecircular array of compression dies one or more times, such as two ormore times, three or more times, four or more times, six or more timeand any range between and including the number of wraps provided. Anexemplary compression die may have a string guide configured along theouter surface to prevent the string from moving off the die and to keepthe string aligned. A string guide may be a recessed area or a pair ofprotrusions spaced apart to allow the string to slide therebetween. Thenumber of revolutions or wraps of the string around the compression diesincreases the mechanical advantage, or the radial force imparted to theproduct for a given string tension. However, the total string-to-diefriction also increases with the number of revolutions, so there arediminishing returns as more revolutions are added. Significant gains inmechanical advantage continue up to about four or five revolutions orwraps. It should also be noted that the actuating string may be designedin various arrangements, such as: removing the pulley, and anchoring oneend the string to the fixed base, while pulling only the other end; orusing two or more segments of string, pulling on one or both ends ofeach segment. It should also be noted that the string may contact thedies in various ways, such as within slots along the outside surface oralong the outer surface, or through holes in the dies, so long aspulling the string and thereby reducing the perimeter of the stringloops causes the central cavity to reduce diameter.

An exemplary radial compression mechanism may comprise a string tensionmechanism that a user interfaces with to apply tension to the string andclose the central cylindrical cavity. An exemplary string tensionmechanism that has a handle for the user to manipulate to tension thestring. An exemplary string tension mechanism may be a lever coupled toa pivot and pivoting the lever may apply tension to the string to closethe cavity. An exemplary string tension mechanism may be a winchmechanism having a winch shaft that the string is attached to. The winchmay have a winch crank that the user can turn or rotate to rotate thewinch shaft and apply tension to the string. The string may wrap aroundthe winch shaft as the central cavity is closed.

An exemplary radial compression mechanism comprises a pulley and thestring may extend around the pulley to the guide the string from thestring tension mechanism to the outer perimeter of the compression dies.A pulley may be configured on an opposing side of the radial compressionmechanism and the string may extend from the string tension mechanism,around the pulley and then around the compression dies and then back tothe string tension mechanism, whereby both ends of the string areattached to the string tension mechanism, such as the winch shaft. Thisarrangement may produce more uniform force to the compression dies asthe string is being pulled or tensioned by both ends. It should also benoted that, although a hand-operated crank is shown here, there willlikely be applications where the winch shaft may be more advantageouslyactuated by, for example, an electric motor or an air motor, with orwithout a gearbox.

A central cavity may have an open diameter that is large enough toreceive an object for compression and the open central cavity diametermay be about 25 mm or more, about 50 mm or more, about 100 mm or more,about 150 mm or more and any range between and including the opencentral cavity diameters provided. A central cavity may be closed to bein a completely closed configuration, or may be limited to a certaindiameter such as about 5 mm, or a diameter just sufficient to adequatelycompress the inserted article.

In an exemplary embodiment, the compression dies are forced open by aspring or springs. A spring or springs may be configured around thecompression dies, such as around the outer diameter and may pull thecompression dies open. As described herein, compression springs may beconfigured between the plurality of dies and the dies may have springseats to retain the springs. It should also be noted that a variety ofmethods may be used move the dies outward, opening the central cavity,when the string is relaxed. These may include: 1) coil-type compressionsprings between the dies, as shown herein, or 2) leaf-type compressionsprings between the dies, or 3) springs between each die and astationary frame member, which may be compression, tension, or torsiontypes. A spring may be a coiled spring as shown or an elastic materialsuch as an elastomer or rubber material that can be elongated orcompressed from an original shape by a load and will then return to saidoriginal shape upon removal of the load.

An exemplary radial compression mechanism including the dies, the baseand the string tension mechanism may be made out of plastic and they maybe injection molded plastic pieces to enable the radial compressionmechanism to be a disposable part, such as the heart valve radialcompression mechanism used in the operating before implantation of manyheart valves.

Accordingly, it is an object of the present invention to provide a newand improved radial compression mechanism.

Another object of the invention is to provide a new and improved radialcompression mechanism for compressing devices, such as large stents andprosthetic heart valves, that require high radial forces.

Another object of the invention is to provide a new and improved radialcompression mechanism that can apply high radial forces and may be builtwith low-cost construction methods.

The summary of the invention is provided as a general introduction tosome of the embodiments of the invention, and is not intended to belimiting. Additional example embodiments including variations andalternative configurations of the invention are provided herein.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention, andtogether with the description serve to explain the principles of theinvention.

FIG. 1 shows a perspective view of the receiving side of an exemplaryradial compression mechanism in a partly-open configuration.

FIG. 2 shows a perspective view of the receiving side of an exemplaryradial compression mechanism in a closed configuration.

FIG. 3 shows a perspective view of the receiving side of an exemplaryradial compression mechanism in a partly-open configuration and with abase member removed to reveal the string wrapped around the outerperimeter of the compression dies.

FIGS. 4 and 5 show perspective views of an exemplary compression diehaving a working surface, outer surface and a spring seat.

FIGS. 6 to 8 show compression dies configured for movement along varioustypes of paths to close the central cylindrical cavity.

Corresponding reference characters indicate corresponding partsthroughout the several views of the figures. The figures represent anillustration of some of the embodiments of the present invention and arenot to be construed as limiting the scope of the invention in anymanner. Further, the figures are not necessarily to scale, some featuresmay be exaggerated to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a representative basis forteaching one skilled in the art to variously employ the presentinvention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

As used herein, the terms “comprises,” “comprising,” “includes,”“including,” “has,” “having” or any other variation thereof, areintended to cover a non-exclusive inclusion. For example, a process,method, article, or apparatus that comprises a list of elements is notnecessarily limited to only those elements but may include otherelements not expressly listed or inherent to such process, method,article, or apparatus. Also, use of “a” or “an” are employed to describeelements and components described herein. This is done merely forconvenience and to give a general sense of the scope of the invention.This description should be read to include one or at least one and thesingular also includes the plural unless it is obvious that it is meantotherwise.

Certain exemplary embodiments of the present invention are describedherein and are illustrated in the accompanying figures. The embodimentsdescribed are only for purposes of illustrating the present inventionand should not be interpreted as limiting the scope of the invention.Other embodiments of the invention, and certain modifications,combinations and improvements of the described embodiments, will occurto those skilled in the art and all such alternate embodiments,combinations, modifications, improvements are within the scope of thepresent invention.

Referring to FIGS. 1 to 3, an exemplary radial compression mechanism 10comprise a plurality of compression dies 20 coupled to a base 30 andarranged in a circular array around central cylindrical cavity 28 forreceiving an object for compression. The base comprises a first basemember 30 and second base member 30′ on opposing sides of thecompression dies; each of the base members comprising die-guiding slots34 that constrain the dies to move approximately radially inward. Basemember 30′ has a receiving aperture for receiving an object into thecentral cylindrical cavity for compression. Ten compression dies arearranged in a circular array forming a cylindrically-shaped centralproduct-receiving cavity. Each die has an inward-facing working surface22 that cooperates with the other dies to form the central cylindricalcavity, and an outer surface 24 that forms the outer perimeter 26 of thearranged plurality of compression dies. FIG. 1 shows the radialcompression mechanism in an open position with the compression dies opento form an open cavity diameter. FIG. 2 shows the radial compressionmechanism in a closed position with compression dies in a closed to forma closed cavity diameter. A string 50 is wrapped around the outerperimeter 26 of the compression dies and is coupled with a stringtension mechanism 60. The exemplary string tension mechanism is a winchmechanism 70 having a winch shaft 72 that rotates to apply tension tothe string. The first end 52 of the string is attached to the winchshaft. A user may rotate the winch crank by the winch handle 76 to applytension to the string and close the central cylindrical cavity 28. Astring is wrapped around the array of dies, contacting each die on itsouter face 24. In an exemplary embodiment the string 50 makes two ormore complete revolutions around the array of dies. When the string ispulled, the string tension results in an inward force appliedapproximately equally to all the dies, causing the dies to move inwardalong the paths defined by the die-guiding slots 32 to decrease thediameter of the central cavity 28. The radial force applied to a productin the central cavity is roughly proportional to the tension in thestring. The string contacts the dies over a relatively broad surfacearea, thereby providing a means to impart high radially inward forces tothe dies, those forces in turn being transmitted from the dies to theproduct in the central cavity, By comparison with prior-art mechanismsthat transmit forces through pins or bearing balls, the presentinvention can impart higher radial forces without damage to themechanism. As shown in FIG. 1, a string guide 58 is configured along theouter surfaces of the dies to prevent the string from slipping off ofthe dies.

As shown in FIG. 3, the radial compression mechanism 10 has one of thebase members removed revealing the actuation string encircling the outerperimeter 26 of the compression dies 20. The string 50 extends aroundthe compression dies, around a pulley and back to the string tensionmechanism 60. The string extends from the winch shaft 72, around thepulley 90 and is then wrapped around the compression dies beforereturning to the winch shaft. Both the first end 52 and the second endof the string 54 are attached to the winch shaft. The string is pulledby winding two ends of the string onto the rotatable winch shaftactuated to the winch crank 74. The pulley changes the direction of oneof the two string ends, so that the two ends of the string approach thearray of dies from opposite directions. This arrangement may provide amore uniform force on the plurality of compression dies.

As shown in FIG. 3, the working surfaces 22 intersect with each other,or nest with the adjacent die's working surface to produce the centralcylindrical cavity 28. As the dies move in unison within the guidingslots 32, the diameter of the central cavity is changed. The guidingslots are shaped so that the dies move along a path that maintains avery small die-to-die gap at the periphery of the central cavity. Theworking surface and the slots may be designed to make the die-to-die gapany size and any function of central cavity diameter. depending on therequirements of the application, but for most applications it should beas small as possible while preventing direct rubbing contact of the dietips, in accordance with the achievable manufacturing tolerances of theparts. In some applications, direct rubbing contact of the adjacent dietips may be allowable or desirable.

As shown in FIG. 3, compression springs 40 are placed between the dies,pushing the dies outward against the string, and causing the centralcavity to open as the string is relaxed. The springs produce a slighttension in the string to prevent it leaving the correct position. Thedies may have spring seats 29, a recess or protrusion from the die toretain the spring in position. As the string is pulled and the centralcavity is closed, there is friction between the string and each of thedies as the string slides across the outer surface 24 of the dies. Suchfriction limits the force that is transmitted to the product and shouldbe minimized. In the preferred embodiment, the string is made ofbraided, fibrous, ultrahigh molecular weight polyethylene, which has avery low coefficient of friction against most materials.

Referring now to FIGS. 4 and 5, an exemplary compression die 20 has alength from a working surface 22 to an outer surface 24, The compressiondie has a spring seat 29, a recess to retain the spring in position. Theexemplary compression die has a width from a first face 21 to anopposing second face 23. The faces of the die are curved and have ageometry to allow nesting of the dies to produce the die array formovement about the central cylindrical cavity. The exemplary compressiondie has a depth from a first side 25 to a second side 27, which definesthe depth of the central cylindrical cavity. Also, the exemplary die hasa pair of guide post 34 extending from each side. The guide posts areconfigured for coupling with the die-guiding slots, such as by beinginserted into the die-guiding slots.

In the preferred embodiment, the guiding slots have curvature directionopposite to that of a spiral shape, constraining the dies to move inrotation about a virtual hinging point may lie outside the envelope ofthe die, but lies within the wedge-like shape formed by the workingsurface and the “back” surface. This results in a “backward” curvingmotion of the dies, as shown in FIG. 6. In this embodiment, the workingsurfaces must have a slightly convex shape to maintain near zerodie-to-die gap throughout the opening and closing motion. Thisembodiment is also an embodiment of the mechanism described in U.S. Pat.No. 8,220,307.

In another embodiment, the guiding slots are linear, constraining thedies to move in a linear path, as shown in FIG. 7. In this embodiment,the working surfaces must be flat to maintain near-zero die-to-die gapthroughout the opening and closing motion.

In another embodiment, the guiding slots have curvature in the samedirection as a spiral shape, constraining the dies to move in rotationabout a virtual hinging point may lie outside the envelope of the die,but lies opposite to the wedge-like shape formed by the working surfaceand the “back” surface. This results in a “forward” curving motion ofthe dies, as shown in FIG. 8. In this embodiment, the working surfacesmust have a slightly concave shape to maintain near zero die-to-die gapthroughout the opening and closing motion. This embodiment is also anembodiment of the mechanism described in U.S. Pat. No. 7,963,142.

It should also be noted that a winch shaft or drum is only one possiblemethod for pulling one or more ends of the string. Other designs arepossible, including, but not limited to: 1) manually pulling the stringswith a human hand, or 2) using any kind of commonly available linearactuator, such as an air cylinder or electric motor with lead screw, or3) gripping and pulling the string with pinch rollers or capstans.

Thus, a new and novel radial compression mechanism has been disclosed.The new and novel radial compression mechanism is constructed with astring or cable wrapped around the array of dies, the tension in thestring causing radially-inward force on the dies and the product in thecavity. Therefore, the limited force capacity of prior art devices hasbeen overcome.

It will be apparent to those skilled in the art that variousmodifications, combinations and variations can be made in the presentinvention without departing from the scope of the invention. Specificembodiments, features and elements described herein may be modified,and/or combined in any suitable manner. Thus, it is intended that thepresent invention cover the modifications, combinations and variationsof this invention provided they come within the scope of the appendedclaims and their equivalents.

What is claimed is:
 1. A radial compression mechanism comprising: a) abase; b) a plurality of compression dies arranged in a circular arrayabout a central axis, each comprising: i) an inward facing workingsurface; and ii) an outer surface; wherein said plurality of compressiondies form a central cavity having a cylindrical shape defined by theworking surfaces; c) a string wrapped one or more revolutions about thecircular array of the plurality of compression dies that contacts eachof the plurality of compression dies; wherein the plurality ofcompression dies are coupled to said base and configured to move inunison from an open position, wherein the central cylindrical cavity hasan open cavity diameter, radially inward to a closed position whereinthe cylindrical cavity has a closed cavity diameter; wherein pulling ofthe string forces the plurality of compression dies to move in unisonfrom said open position to said closed position; and wherein said opencavity diameter is larger than said closed cavity diameter; wherein theradial compression mechanism is configured to produce a radially inwardcompression force along the working surfaces of the central cavity froman open position to a closed position; and wherein the central cavitymaintains the cylindrical shape from said open position to said closedposition.
 2. The radial compression mechanism of claim 1, wherein thestring is wrapped four or more times around the circular array of theplurality of compression dies.
 3. The radial compression mechanism ofclaim 1, further comprising a string tension mechanism, wherein thestring is coupled to the string tension mechanism and wherein actuatingthe string tension mechanism pulls the string to move the plurality ofcompression dies radially inward to close the central cylindricalcavity.
 4. The radial compression mechanism of claim 3, wherein thestring has a first end and a second end and wherein both the first andsecond ends of the string are coupled to the string tension mechanism.5. The radial compression mechanism of claim 4, further comprising apulley and wherein the string extends from the string tension mechanismaround the pulley and then around the plurality of compression dies. 6.The radial compression mechanism of claim 3, wherein the string tensionmechanism is a winch mechanism comprising: a) a winch shaft; b) a winchcrank; wherein the string is coupled to the winch and wherein actuatingthe winch pulls the string to move the plurality of compression radiallyinward to close the central cylindrical cavity.
 7. The radialcompression mechanism of claim 6, wherein the string has a first end anda second end and wherein both the first or second ends of the string arecoupled to the winch mechanism.
 8. The radial compression mechanism ofclaim 7, further comprising a pulley and wherein the string extends fromthe string tension mechanism around the pulley and then around theplurality of compression dies.
 9. The radial compression mechanism ofclaim 3, wherein the string tension mechanism is a lever mechanismcomprising: a) a lever pivot; b) a lever arm that rotates about thelever pivot; wherein the string is coupled to the lever and whereinactuating the lever pulls the string to move the plurality ofcompression radially inward to close the central cylindrical cavity. 10.The radial compression mechanism of claim 9, wherein the string has afirst end and a second end and wherein both the first or second ends ofthe string are coupled to the lever mechanism.
 11. The radialcompression mechanism of claim 10, further comprising a pulley andwherein the string extends from the string tension mechanism around thepulley and then around the plurality of compression dies.
 12. The radialcompression mechanism of claim 1, wherein the string has a first end anda second end and wherein the string is wrapped around the circular arrayof the plurality of compression dies with at least one of the first orsecond ends coupled to the base.
 13. The radial compression mechanism ofclaim 1, further comprising a spring coupled to the plurality ofcompression dies to force the central cylindrical cavity open.
 14. Theradial compression mechanism of claim 1, further comprising a springconfigured between each of the plurality of compression dies to forcethe plurality of compression dies apart and the central cylindricalcavity open.
 15. The radial compression mechanism of claim 14, whereineach of the plurality of compression dies comprise a seat to receive andretain an end of said spring.
 16. The radial compression mechanism ofclaim 1, comprising at least three compression dies.
 17. The radialcompression mechanism of claim 1, wherein each of the plurality of dieshas an outer surface and wherein the string extends around said outersurface of each of the plurality of dies.
 18. The radial compressionmechanism of claim 1, wherein the plurality of dies are backward curvingdies.
 19. The radial compression mechanism of claim 1, wherein theplurality of dies are linear dies.
 20. The radial compression mechanismof claim 1, wherein the plurality of dies are forward curving dies.